Project Summary/Abstract A hallmark of the humoral immune response is a two phased antibody response, the first being rapid and providing for low affinity antibodies, and the second occurring with a week delay but providing high affinity antibodies. An appropriate balance of both phases of the response is critical for an effective immune response. The two phased humoral immune response is governed by B-cell population dynamics that represent the composite of the decisions made by individual cells whether to enter a growth phase and the cell cycle that results in division, whether to survive or die, and whether to differentiate into antibody secreting plasma cells and/or memory B-cells. At any given timepoint there is a great variety of B-cell fates, and prior studies assumed that this is due to stochastic fate decisions by individual cells at each generation. Instead, our recently established long-term microscopy workflow revealed that cells make highly deterministic fate decisions, and that the cell-to-cell variability within the population is largely due to heterogeneity in the founder cells. This renders humoral immunity substantially more predictable, so long as we have a mechanistic understanding of how molecular networks control B-cell decision making in proliferation and differentiation. The overarching goal of the proposed project is to develop quantitative understanding and multi-scale model of how the multi-dimeric NF?B system controls B-cell decision making to effect cell survival, proliferation, and differentiation. The overarching hypothesis of the proposed studies is that the coordinated dynamics of NF?B family members RelA and cRel control the phasing of B-cell proliferation and differentiation and thus the affinity, abundance and diversity of antibodies and hence efficacy of the humoral immune response. We will address this hypothesis with an iterative systems biology approach structured into following three Specific Aims: 1. Delineate how NF?B system dynamics control the lineages of proliferating B-cells 2. Delineate how NF?B system dynamics control plasma B-cell differentiation 3. NF?B system control of humoral immunity in vivo: phasing low and high affinity antibody responses Each Aim involves novel multiscale mathematical modeling and quantitative experimentation, including unprecedented long term microscopy, novel fluorescent reporter mouse strains, and single cell genomic technologies.